1. Technical Field
The present invention relates to coded modulation and more particularly to non-binary low-density parity-check (NB-LDPC) mode-multiplexed four-dimensional signaling based on orthogonal frequency division multiplexing (OFDM).
2. Description of the Related Art
Due to the rapid growth of data-centric services and the general deployment of broadband access networks in recent years, there has been an elevated demand driving the dense wavelength division multiplexing (DWDM) network upgrade from 10 Gb/s per channel to more spectrally-efficient channel transmission rates (e.g., 40 Gb/s, 100 Gb/s, and beyond). However, as the symbol rate increases, the deteriorating effects of linear and nonlinear fiber impairments are known to exacerbate. For example, as the communication rate over a given medium increases, transmission becomes increasingly sensitive to errors due to various linear and nonlinear channel impairments such as chromatic dispersion, PMD and fiber nonlinearities. The Shannon limit for a noise-influenced channel describes a maximum amount of error-free data that can be transmitted with a specified bandwidth—it is therefore helpful to have robust codes and modulation schemes that closely approach the Shannon limit without imposing high requirements in terms of implementation cost and complexity.
Bit interleaved (BI) low-density parity-check (LDPC) coded modulation (CM) based on large girth LDPC codes provides excellent performance, but requires code rate and bandwidth to increase in order to compensate for information loss due to coding. Meanwhile, quasi-cyclic (QC) LDPC codes are easy to implement, but this comes at the expense of performance. Large-girth QC-LDPC codes provide good bit-error rate (BER) performance, but require excessive codeword length for larger girths, as the code rate, and therefore the bandwidth, must increase to compensate for information loss due to coding. Another approach to resolve channel impairments is to employ advanced techniques in modulation, detection, coding, and signal processing. For example, space division multiplexing has been employed to achieve multi-Tb/s serial optical transport over few-mode-fibers (FMF). However, there exist additional channel impairments in FMFs that do not exist in single-mode-fibers (SMF). These impairments (e.g., linear and non-linear coupling among spatial modes) provide an additional challenge to serial multi-terabit long-haul transmissions.